1. Field of the Invention
The invention relates to a sealed battery in which an electrode body is housed together with an electrolyte inside a sealed case. More particularly, the invention relates to a sealed battery provided with a safety valve that opens when the internal pressure of the case rises. The invention also relates to a vehicle provided with this sealed battery.
2. Description of the Related Art
Lithium-ion batteries and other sealed secondary batteries have become increasingly important in recent years as onboard power supplies for vehicles and power supplies for personal computers and portable terminals and the like. In particular, lithium-ion batteries, which are lightweight and capable of obtaining high energy density, are also used as onboard high output power supplies for vehicles. These sealed batteries are typically provided with an internal pressure release mechanism for releasing the gas pressure inside the case (i.e., the internal pressure) when it rises excessively due to overcharging or the like. Japanese Patent Application Publication No. 11-273640 (JP-A-11-273640), Japanese Patent Application Publication No. 2001-102024 (JP-A-2001-102024), and Japanese Patent Application Publication No. 2001-256944 (JP-A-2001-256944), for example, describe one representative example of such an internal pressure release mechanism that is a so-called safety valve that is made by forming a thin portion (typically made of metal) that is thinner than any other part of the case in a portion of the case. This safety valve is structured such that when the internal pressure of the case becomes equal to or greater than a predetermined value (i.e., an opening pressure), the thin portion fractures (opens) to release that internal pressure.
FIG. 15 shows one example of the structure of the safety valve formed in a portion of a sealed battery case. The safety valve 2 shown in the drawing is formed from a thin portion 3 having a horizontally long shape (more specifically, an oval shape in which both sides in the longer direction are semicircles) and thinner than any other part of the battery case, and a fracture groove portion (an impressed portion) 4 formed in the thin portion 3. If the internal pressure of the case becomes equal to or greater than a predetermined value (i.e., an opening pressure), that internal pressure will cause the fracture groove portion 4 to fracture and the thin portion 3 will typically turn outward, thereby opening the valve, as shown in FIG. 16.
With the safety valve 2 described above, the area of the open portion S of the thin portion 3 that opens tends to be limited, as shown in FIG. 16. In particular, it is difficult for the openings at both end portions in the longer direction of the horizontally long shaped (or vertically long shaped) thin portion to open. However, with this type of safety valve, when the internal pressure of the case rises above a predetermined level, it is necessary to open the valve as quickly as possible and open the thin portion wider in order to quickly release the gas generated in the case outside the battery. However, if a surplus of fracture groove portions are formed in the thin portion or the depth of the fracture groove portion is made excessively deep (i.e., if the groove portion is made extremely thin) in order to accomplish this, the safety valve may open before the internal pressure of the case exceeds the predetermined level (such as when the internal pressure rises abnormally).
Therefore, forming a surplus of the excessive fracture groove portions in the thin portion or making the depth of the fracture groove portion excessively deep (i.e., making the groove portion extremely thin) may also result in variation in the internal pressure of the case at which the valve opens among individual batteries (among safety valves provided in individual batteries). Also, if the thin portion is formed too large, it May reduce the strength of the case and the thin portion may break with sudden vibration or impact.